Microporous polyolefin membranes are widely used in various applications such as battery separators, electrolytic capacitor separators, various filters, moisture-permeable, waterproof clothes, reverse osmosis membranes, ultrafiltration membranes, microfiltration membranes, etc.
It has been known in the art that the microporous membrane may be produced by forming a molten mixture of a polyolefin, an organic solvent and inorganic powder such as silica fine powder into a sheet, and extracting the organic solvent and the inorganic powder from the sheet. However, because the inorganic powder should be extracted in this method, it is difficult to adjust the permeability of the resultant microporous membrane, which largely depends on the particle size of the inorganic powder, to a desired level.
Various methods of producing a microporous membrane from an ultra high-molecular-weight polyolefin have been recently proposed in Japanese Patent Laid-Open Nos. 60-242035, 61-195132, 61-195133, 63-39602, 63-273651, etc. In these methods, a solution prepared by dissolving an ultra high-molecular-weight polyolefin having a weight-average molecular weight of 7.times.10.sup.5 or more in a non-volatile solvent while heating is formed into a gel-like sheet, whose non-volatile solvent content is adjusted by removing part of the non-volatile solvent. The gel-like sheet is then stretched while heating, and the residual non-volatile solvent is removed from the stretched sheet by extraction to produce the microporous membrane.
In the methods mentioned above, a large number of fine pores are formed by stretching the gel-like sheet after solidification by cooling. Therefore, the microporous membrane produced by these methods is characterized by a small pore size and a narrow pore size distribution. However, these methods fail to provide microporous polyolefin membranes having relatively large pore sizes and high permeability suitable for high-precision filtration membranes, battery separators, etc.
In such circumstances, the inventors found that a microporous polyolefin membrane having excellent permeability can be produced by preparing a solution of a polyolefin composition comprising ultra high-molecular-weight components, extruding the solution through a die lip of an extruder into a sheet, rapidly cooling the extruded sheet to form a gel-like sheet, and removing the residual solvent therefrom preferably without stretching. However, because the polyolefin composition has a high weight-average molecular weight, it is difficult to prepare a high-concentration polyolefin composition solution for forming the gel-like sheet. Thus, this method takes too much time to produce the microporous membrane, posing poor production efficiency. Further, the gel-like sheet is likely to have unsatisfactory surface conditions, with poor formability.
Accordingly, an object of the present invention is to provide a method of easily and efficiently producing a microporous polyolefin membrane having a relatively large pore size and excellent permeability.